Strange choice of words

[frozen-jelly.livejournal.com]

While reading Sirius' latest entry, I noticed this sentence:
"Nothing's worse for a flu than getting up one's Irish (and, er, Molly's got more than most, old man)."

I have absolutely no idea what this is meant to mean. I can only guess it is some vaguely racist use of the word Irish to mean angry or frustrated, which is very odd. Can anyone shed light on this usage, I have never heard it before? It really makes very little grammatical sense.